Current sensing circuitry

Abstract

A current sensing circuit arrangement for controlling the response of a circuit electrically isolated from the circuit in which sensed current is flowing is disclosed. The input portion of the circuit arrangement provides paths for D.C. input current, one through a current sensing element and another through a light emitting diode circuit. The current-to-voltage characteristic of the current sensing element is such that changes in voltage across the element are substantially less than proportional to changes in current through the element. The light emitting diode circuit is connected in parallel with the current sensing element and produces light energy dependent on the voltage across the element. Light energy so produced is received by an electrically isolated light sensitive semiconductor device which device is for controlling the response of an output control circuit.

I claim: 
     
       1. A current sensing circuit arrangement, comprising: (a) a plurality of series connected diodes providing a first D.C. current path from a first point to a second point so that current flows in the forward direction through each of said diodes when a positive voltage is applied to said first point relative to said second point;   (b) a light emitting diode circuit connected in parallel with said series connected diodes for producing light energy dependent on the voltage across said current sensing element when current flows in said first path from said first point to said second point;   (c) a light sensitive transistor electrically isolated from said light emitting diode circuit and positioned to receive said light energy produced, said light sensitive transistor having a base for receiving said light energy, an emitter and a collector;   (d) an output control circuit, comprising: (i) a control circuit input operatively connected to said light sensitive transistor;   (ii) a control circuit output;   (iii) a high input impedance, high gain amplifier having an amplifier input and an amplifier output, said amplifier output operatively connected to said control circuit output;   (iv) voltage reference means for providing a fixed voltage reference at a voltage reference point;   (v) first resistance means connected between said control circuit input and said amplifier input;   (vi) second resistance means and a blocking diode connected in series between said amplifier input and said voltage reference point.     
     
     
       2. A current sensing circuit as defined in claim 1 wherein said first resistance means and said second resistance means comprise respective portions of a resistor having a voltage tap between said portions, said tap being connected to said amplifier input and being adjustable in position along said resistor to vary the relative resistance of said portions. 
     
     
       3. A current sensing circuit as defined in claim 1 wherein said light emitting diode circuit comprises a light emitting diode and resistance means connected in series therewith for limiting the maximum current through said light emitting diode, said light emitting diode and resistance means providing a second D.C. current path from said first point to said second point, said light emitting diode being disposed to conduct current in the forward direction and produce said light energy when current flows in the second path from said first point to said second point. 
     
     
       4. A current sensing circuit as defined in claim 2 wherein said light emitting diode circuit comprises a light emitting diode and resistance means connected in series therewith for limiting the maximum current through said light emitting diode, said light emitting diode and resistance means providing a second D.C. current path from said first point to said second point, said light emitting diode being disposed to conduct current in the forward direction and produce said light energy when current flows in the second path from said first point to said second point. 
     
     
       5. A current sensing circuit as defined in claim 1 wherein the voltage at said amplifier output switches from a first discrete value to a second discrete value when current flowing through said current sensing circuit from said first point to said second point drops below a predetermined threshold level.